This invention relates to radio receivers, and particularly but not exclusively to such receivers forming part of the transceiver of a portable radio telephone or handset, for communication with a base station in a radio telephone system. The invention also relates to a method of operation of such radio receivers. The invention also relates, particularly but not exclusively, to handsets for communication with base stations in a digital cellular radio telephone system employing transmission by a plurality of carrier frequencies in frames, such as TDMA frames, each consisting of a predetermined number of time slots.
Any radio system in which information is transmitted in bursts causes problems for the receiving station in maintaining synchronicity with the signals received. Many protocols provide for a solution to this problem by including a synchronisation field at the start of a burst to allow the receiving station to synchronise with the signal before signals carrying data or other information are received.
In digital systems the receiver must achieve both `lock recovery` or bit synchronisation with the incoming signal allowing data to be recovered, and slot, or burst, synchronisation that allows the receiver to determine which point of a burst is being received. For this purpose, a synchronisation field comprising a known string of bits is provided at the beginning of each burst so that the receiving station can correlate its timing to that of the incoming signal. The known string allows bit synchronisation to be achieved relatively quickly in good conditions i.e. when the signal is of a high quality. The bit synchronisation field must, however, be long enough for bit synchronisation to be achieved under all the circumstances in which the signal quality is suitable to provide information. Following the bit synchronisation string is typically a slot synchronisation string that allows the receiving station to locate a particular point in the burst so that the category or type of information carried by the subsequently received bits of that burst can be identified by virtue of their position within the burst.
Signalling protocols typically provide for error detection so that good data received can be distinguished from corrupt data. The quality of a channel for receiving signals can be monitored to determine if a new channel is necessary in order to preserve transfer of the information carried by the signal bursts from the base station to the handset or vice versa. It is desirable to monitor channel quality to allow a better quality channel to be used before too much information is lost. The quality of a received signal can be used as a trigger for changing channel.